SEED-BEARING PLANTS 



429 



filling, the venter of the archegonium. The pollen- tube 

 passes between the neck-cells of the archegonium, but 

 does not ordinarily enter the venter. The apex of the 

 tube is ruptured, probably by internal osmotic pressure, 

 and its entire contents are emptied into the cytoplasm 

 of the egg. One of the sperm-nuclei unites with the 

 egg-nucleus (June of the second season), and fertilization 

 is accomplished (Fig. 319). 



Fig. 319. —White pine {Pinus Sir oh us). Longitudinal section through 

 an archegonium at the time of fertilization. Above the -fusing nuclei are 

 various other elements emptied into the egg from the pollen-tube. Col- 

 lected June 21, 1898. X about 62. s.g, starch grains; p.r, prothallium; 

 c.p.t, cytoplasm from pollen-tube; st.c, stalk-cell; t.n, tube-nucleus; s.n, 

 sperm-nucleus; e.n^ egg-nucleus; n.s, nutritive spheres. (After Margaret 

 C. Ferguson.) 



382. Formation of the Seed. — After fertilization the 

 oosperm begins at once to develop, giving rise to three 

 distinct structures; the pro-embryo, the suspensor, and 

 the embryo-sporophyte. During the early divisions of 

 the fertilized egg the male and female chromatins can be 

 clearly distinguished (Fig. 320). At the same time the 

 adjacent tissues of the ovule become transformed. A 

 portion of the prothallus or gametophyte nourishes the 

 developing embryo, but the large bulk of it becomes 



